He was America’s Top Cop for five decades, but somewhere along the line, the FBI founder went from protector to tyrant in the minds of many Americans.

Hoover unified the federal police force, helped popularize fingerprints and other forensic evidence… then went sadly astray, abusing his power, manipulating elected officials and, most damningly, wiretapping and intimidating civil rights leaders. That tragic arc is at the heart of director Clint Eastwood’s drama, starring Leonardo DiCaprio as the notorious G-man.

“I started to become curious about the ‘why,’” says screenwriter Dustin Lance Black, Oscar-winner for 2008’s Milk. “He did wonderful things for this country, but why did he ended up doing so many things that were heinous and harmful? I think it was all in the name of trying to fill that void, where love goes, with public admiration. This is a guy who from his earliest years was told he could never express the love he felt.”

Was Hoover was gay or straight? The movie leaves it open to interpretation, according to Eastwood, but one thing is undeniable – he was an achingly solitary man. And there was far more to him than that.

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“Well, I grew up with Hoover in the 30’s and 40’s as The G-man,” says Eastwood. “He was always the authority; always sort of the top cop. Certainly the FBI was considered somewhat revered, and [we were] somewhat in awe, I guess. When you talk to FBI agents now who’ve worked with him, they all have different opinions of him. A lot of them liked Hoover very much, liked working for him very much.”

The movie J. Edgar tracks him from his early days, building the FBI into an actual law enforcement agency, instead of the powerless entity they were before.

“They were just guys going around asking questions. Almost like reporters, trying to create a story to give to the localized police, and the police are the ones that had all the power,” says DiCaprio. “It was almost like our country was fractioned off into little city-states that have their own jurisdiction, and here comes a man that creates a federal police force where you’re held accountable in our country for your actions and your deeds.”

He also pushed for a federal fingerprints database, and helped popularize the use of scientific evidence to convict criminals. “We’re not simply relying on testimony from eyewitnesses that can often be misleading and can take you in different directions,” DiCaprio says. “He brought forensics into it. The modernized way that we convict people and people are put on to trial and the evidence we use is all a result of the way J. Edgar Hoover designed the FBI.”

Then of course, after amassing so much power, things went astray …

“Here’s this guy starting the Bureau of Investigation, which later became the Federal Bureau of Investigation, and then goes on to stay for 48 years through eight presidents,” Eastwood says. “Of course nobody could fire him, because he had something on everybody.”

But what did they have on him?

Practically nothing, actually – apart from rumor and innuendo.

“He knew what he could do to everybody else by finding out what their life was like, but on the same token, everything in his life was pretty much kept between he and Helen Gandy and Clyde Tolson,” said Eastwood, referring to the Associate FBI Director, rumored to be Hoover’s lover, played by The Social Network’s Armie Hammer (pictured above.) Gandy, Hoover’s secretary, is played in the film by Naomi Watts, filling out that secretive FBI triumvirate.

While the screenwriter, Black, believes it was Hoover’s suppressed sexuality that twisted him into the ultimate control freak with ultimate enforcement power, the screenplay keeps things somewhat ambiguous. After all, the premise is that failing to be able to feel something for another person is what warped Hoover, leading him to see everyone from Martin Luther King Jr. to Robert F. Kennedy as enemies of the state.

Eastwood says he didn’t want to make a definitive statement about whether Hoover was gay, since that history is far from certain: “Some people might interpret it that way. Some people might say [he and Tolson] were just inseparable pals. Or maybe it’s a love story without being gay, I don’t know. But it’s very interesting, the way Lance laid out the script. It was nicely written. It didn’t go to the obvious.”

DiCaprio plays Hoover into his bulldog-esque later years with prosthetic makeup, though it was never easy getting into the man’s skin, especially as the lawman became the outlaw.

“I think that he was always an outsider,” says the actor. “I think that’s what this story is about, how he sort of lost touch towards the end of his life, to say the least.” By the time he died, DiCaprio says, Hoover was obsessed with “pretty far-fetched, bizarre, highly paranoid stuff.”

Oh, Leo, if Hoover were alive today, you’d be so wire-tapped right now.